Chromosomal jumping has been proposed as a rapid technique for ordering large segments of the genome, increasing the speed at which genes associated with hereditable diseases of man can be isolated. Technical difficulties associated with the construction of the necessary jumping and junction libraries has slowed the successful aspplication of this technique. The most difficult step in the construction of jumping libraries is circularizing extremely large genomic DNA fragments onto a selectable marker in a single reaction. Strategene has developed a novel cloning technology which allows this step to be reduced to a simple circularization without the selectable marker. The selectable marker is added at a later step under conditions which highly favor the desired ligation products. We also include the additional feature of T3 and T7 RNA polymerase promoters positioned to generate high specific activity probes without subcloning or fragment isolations. Strategene is well qualified for this project since its success depends primarily the construction a vector system which represents a simple modification of preexisting vectors previously developed by Stratagene.